With development of information communication technologies, a variety of wireless communication technologies have been developed. A wireless local area network (WLAN) is a technology permitting wireless access to Internet in specific service areas such as home or companies or air planes by the use of portable terminals such as a personal digital assistant (PDA), a laptop computer, and a portable multimedia player (PMP) on the basis of a radio frequency technology.
These days, thanks to the increased availability of WLANs, portable terminal users such as laptop computer users are able to perform their tasks with increased mobility. For example, a user can take his laptop computer from his desk into a conference room to attend a meeting and still have access to his local network to retrieve data and have access to the Internet via one or more modems or gateways present on the local network without being tethered by a wired connection. Similarly, business travelers commonly use their portable terminals to gain access to their email accounts, to check if there is any unread email, and to read and send email.
In the initial WLAN stage, a data rate of 1 to 2 Mbps was supported by the use of frequency hopping, spread spectrum, and infrared communication using a frequency of 2.4 GHz. In recent years, with the development of the wireless communication technology, a data rate of 54 Mbps can be supported by applying the orthogonal frequency division multiplex (OFDM) technology, etc. to the WLAN. In addition, the WLAN has developed or is developing wireless communication technologies for improvement in quality of service (QoS), compatibility of an access point (AP) protocol, security enhancement, wireless resource measurement, wireless access in vehicular environment, fast roaming, mesh network, inter-working with external networks, wireless network management, and the like.
In the system based on Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, a basic service set (BSS) means a set of stations (STAs) successfully synchronized. A basic service area (BSA) means an area including members of the BSS. The BSA can vary depending on propagation characteristics of a wireless medium. The BSS can be basically classified into two kinds of an independent BSS (IBSS) and an infrastructured BSS. The former means a BSS that constitutes a self-contained network and that is not permitted to access a distribution system (DS). The latter means a BSS that includes one or more access points (APs) and a distribution system and that uses the APs in all the communication processes including communications between the stations.
Before the IEEE 802.11e standard which is published on 11Nov. 2005 and is incorporated by reference, the infrastructured BSS required to transmit data necessarily through the AP without permitting the direct transmission of data between non-AP stations (non-AP STAs). STAs are not allowed to transmit frames directly to other STAs in a BSS and should always rely on the AP for the delivery of the frames. The IEEE 802.11e standard agreed a direct link setup (DLS) between the non-AP STAs to improve the efficiency of WLAN. A QBSS is a BSS supporting quality of service (QoS). A QSTA is a STA supporting QoS. A legacy STA is a STA not supporting QoS. A QAP is a AP supporting QoS. The QBSS includes QSTAs and one or more QAPs. QSTAs may transmit frames directly to another QSTA by setting up such data transfer using DLS. Non-AP QSTAs can set up a direct link and directly communicate with each other through the direct link. The direct link is a unidirectional link from one non-AP QSTA to another non-AP QSTA operating in the same infrastructure QBSS that does not pass through a QAP. Once the direct link has been set up, all frames between the two non-AP QSTAs are exchanged directly.
To set up the direct link, the two non-AP QSTAs exchanges a DLS request frame and a DLS response frame via the QAP. This means setting up DLS rely on the QAP. The WLAN system currently used is generally based on the IEEE 802.11a/b/g standard. This means QBSS is not the basic BSS. The non-AP STAs may be QSTAs supporting the QoS but the AP may be a legacy AP not supporting the QoS. As a result, non-AP QSTA cannot utilize the DLS service since legacy APs cannot support the DLS service.
STAs can operate in one of two power management modes of an active mode (AM) and a power save mode (PSM). Since the non-AP STA is generally a user's portable device, it is necessary to support the PSM so as to effectively manage the power. In the IEEE 802.11e standard, power management is called as automatic power save delivery (APSD). The APSD defines two delivery mechanisms, namely unscheduled APSD (U-APSD) and scheduled APSD (S-APSD).
In the direct link, power management is also needed. IEEE 802.11e standard provides APSD support in the QBSS. After establishing DLS, one QSTA in AM or PSM transmits a data frame and/or a management frame to the other QSTA in PSM or AM via the direct link. But when non-AP QSTAs and a legacy AP are co-exists, IEEE 802.11e standard does not disclose how to establish the direct link and how to manage PSM.